Mutations in the human connexin 26 gene (Cx26, or GJB2) are the leading cause of nonsyndromic deafness in the United States. Mutations in two additional connexin genes, Cx30 (GJB6) and Cx3l (GJ63), also produce hearing loss in humans. While this illuminates a critical function for cochlear gap junctions, it is not clear how a common pathology can arise from mutations within different connexin genes that have an overlapping expression pattern in the inner ear, as is the case for Cx26, Cx30 and Cx3l. There are no gap junctions present between the sensory hair cells in humans; rather they are expressed in the supporting cells of the cochlea. The current hypothesis is that these junctions play a role in the re-circulation of potassium ions between the endolymph and perilymph. It is difficult to reconcile this model with the available data on potassium permeation through gap junction channels, as all connexins are readily permeated by this cation and the loss of a single cochlear connexin would still leave two functional connexins available to perform this task. Connexins do show differential permeability to a wide range of other small molecules and second messengers, and we hypothesize that these permeation differences are critical for cochlear function, and more difficult to compensate for following the functional loss of one of the three available channel subunits. The objective of this application is to precisely define which permeation properties of Cx26 are necessary for normal auditory function in humans. To achieve this goal, we first propose to screen mutant Cx26 alleles for functional activity in the paired Xenopus oocyte assay. Cx26 mutants that retain channel function will then be transfected into mammalian cell lines, and have their permselectivity properties analyzed by dual patch clamp methods. Contrasting the differences in permeation between wildtype and disease causing variants of Cx26 will not only provide mechanistic insight into hearing loss, but will also provide a general model for the need for connexin diversity in other tissues where human disease results from mutations in connexin genes.